Octyne dissolving metals

Dissolving metal reductions works very well with aldehydes and ketones, but alkenes are not readily reduced under the same conditions. For example, 1-hexene is reduced to hexane in only 41% yield with Na/MeOH/liquid NHg.14 Alkynes, on the other hand, are reduced to alkenes in good yield using dissolving metal conditions, and the experimental evidence shows that the -alkene is the major product. In a typical example, 4-octyne (60) is treated with sodium in liquid ammonia, and oct-4 -ene (64) is isolated in 90% yield. None of the Z-alkene is observed in this reaction. The reaction with sodium in liquid ammonia is an electron transfer process similar to that observed with ketones and aldehydes, but how is the E geometry of the alkene product explained ... 

What can we do if we want the tra s-alkene rather than the ds-isomer from alkyne reduction This can be accomplished using a dissolving metal reduction. When an alkali metal such as sodium is added to liquid ammonia, it is ionized to give solvated electrons (these are blue, but that s a story for the physical chemists...). One electron is added to the alkyne to give a radical anion (Figure 11.94). Because electrons repel each other, the orbitals containing the lone pair and the odd electron are on opposite sides of the triple bond. The lone pair is protonated by the solvent then a further electron and proton are added to complete the process. Thus, 4-octyne is cleanly reduced to fraKS-4-octene. 

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